This application relates to methods for electrostatically depositing a substance on a substrate.
The rapid expansion of supercritical fluid solutions through a small orifice (referred to herein as the xe2x80x9cRESSxe2x80x9d process) produces an abrupt decrease in dissolving capacity of the solvent as it is transferred from a supercritical fluid state, having near liquid density, to a very low density phase after the expansion. This abrupt transition in solvent characteristics results in the nucleation and growth of nanometer-sized particles from any low vapor pressure solute species that are dissolved in the solution prior to expansion. Because the solvent is transformed into the gas phase during the RESS expansion, RESS products are generated xe2x80x9cdryxe2x80x9d since they are substantially free of residual solvent. A long-standing difficulty with the RESS process is that particles in the range from 10-500 nm are difficult or impossible to deposit on a surface since their extremely low mass causes them to remain entrained in the expansion gas.
Electrostatic deposition has been used in connection with spraying of liquid compositions. In such conventional systems, the spray composition is in the liquid state at the spray nozzle exit tip. Mechanical forces (shear forces in the nozzle) cause the breakup of the liquid stream into smaller droplets of at least one micron or larger. Liquid spraying is not a true thin film technique since relatively large particles or agglomerations of molecules actually impact the substrate surface. During the electrostatic charging process at the nozzle tip, charge can be transferred through the liquid or from the nozzle surface to the liquid surface. This charge is then transferred to the individual droplets as they form during the droplet breakup process.
A continuing need exists for environmentally benign methods for producing nanometer-thick films on substrates. Most conventional methods use environmentally problematic volatile organic solvents, do not offer sufficient film thickness and uniformity control, and/or are costly. Methods that can combine the environmental benefits of RESS with the need for uniform nanometer-thick films would be quite useful.
Disclosed herein are methods that can be used to produce thin films or coatings on a substrate. One method embodiment involves forming a supercritical fluid solution of at least one supercritical fluid solvent and at least one solute, discharging the supercritical fluid solution through an orifice under conditions sufficient to form solid particles of the solute that are substantially free of the supercritical fluid solvent, and electrostatically depositing the solid solute particles onto the substrate. One aspect of this embodiment contemplates charging the solid solute particles to a first electric potential and depositing the charged solid solute particles onto a substrate to form a film. Substrates having at least one surface upon which such a film has been deposited are also disclosed.
A further embodiment involves forming a solution of at least one supercritical fluid solvent and at least one solute, discharging the solution through an orifice under conditions sufficient to form particles of the solute having a mean particle size of less than 1 micron, and electrostatically depositing the solute particles onto the substrate.
An additional disclosed embodiment includes forming a mixture of at least one supercritical fluid and about 3.0 weight percent or less of at least one polymer (based on the total weight of the supercritical fluid and the polymer), flowing the mixture through an orifice to produce a spray that includes particles of the polymer, and electrostatically depositing the polymer particles onto the substrate.
Also disclosed are methods for collecting bulk powders that involves collecting the solid solute particles that are electrostatically deposited on a substrate.
The disclosed methods will become more apparent from the following detailed description of several embodiments.